1. Field of the Invention
The invention relates to a method of heat treatment for parts reaching the end of their useful operational life after having suffered damage as a result in particular of creep. The object of the method is to enable them to recover their initial properties so that they may last longer. It relates to machine parts of heat-resistant nickel base alloy comprising a hardening phase .gamma.', and applies in particular to turbomachine rotor blades.
Such blades should be able to withstand high temperature creep as they are mounted on a disc which rotates at between 5,000 and 20,000 rpm, while being exposed to hot gases at between 900.degree. C. and 1300.degree. C., having an oxidising effect, issuing from the combustion chamber. Research has therefore been directed towards cast alloys, whose chemical compositions may be optimized, and which are capable of being substantially hardened by precipitation with a view to improving their resistance to fracture as a result of creep. Nickel-based superalloys used in aircraft engineering have a hardening phase .gamma.' the volumetric fraction of which may reach 70%.
However, in operation, the rotor blades subjected to such mechanical and thermal stresses suffer permanent elongation through creep, which leads inevitably to their systematic scrapping after a certain number of hours of use so as to avoid the danager of a catastrophic fracture. For example, high pressure rotor blades in a certain number of engines have their useful operational life limited to about 800 hours because of creep.
As this creep deformation process results in a degradation of the microcrystalline structure, the invention has for its object the provision of a thermal treatment method permitting the restoration of the initial structure under conditions compatible with the geometrical criteria of the parts.
2. Description of the Prior Art
In the course of earlier work regeneration treatments have been devised. For example, French Pat. No. 2,292,049 describes a process for extending the duration of the secondary creep of some alloys: it consists in a heat treatment without stress, conducted at a temperature below that for dissolving the compounds. This temperature corresponds in practice to the maximum temperature for the operation of the part; moreover, that temperature is held for quite a long time because it has to permit, according to the hypothesis put forward, the annihilation of the gap and cavity networks by means of a diffusion process. This treatment, restricted in temperature terms, is certainly ineffective for parts having operated at high temperatures, such as 1100.degree. C., for it does not permit the regeneration of the microcrystalline structure. Moreover, its duration makes it uneconomical for industrial application.
As a further example, French Pat. No. 2,313,459 relates to a method of improvement in the continued useability of metal parts which have suffered permanent elongation. It consists in subjecting said parts, before surface cracks appear, to a hot isostatic compression or compacting, at a temperature below that at which an enlargement of the grain takes place, and then in applying a re-dissolving treatment of the phases, followed by a hardening annealing. The importance of the compacting lies in the fact that it closes the decoherence caused by creep and closes any remaining pores formed during casting. This technique, however, is rather cumbersome to implement; it is not justified in all cases. Moreover, the subsequent heat treatment does not permit control of the precipitation mechanisms; neither does it take into account a deterioration of the surface protective layer. Finally it is not capable of economical industrial application.
Alloys of this type designed for use at high temperatures exhibit poor corrosion properties beyond 900.degree. C., particularly in a sulfurizing atmosphere; accordingly they require surface protection which may be a nickel aluminizing coating obtained by thermo-chemical means. The problem posed by this type of protection is that any heat treatment of the part at beyond a certain temperature and for more than a certain period of time causes intermetallic diffusion modifying its chemical composition and its properties. To prevent this, it is normally sufficient to effect a preliminary treatment which removes the said layer. But this operation has been found to be impossible on rotor blades provided with internal cooling channels, as it would unacceptably reduce their already thin wall thickness.
The invention has therefore as its second object the provision of a heat treatment which does not require the preliminary operation of removal of the protective layer.